Procedures are being developed for analysis of N15 and C13 in biologically important substances by nuclear magnetic resonance spectroscopy. The methods are nondestructive and applicable to mixtures of compounds. The major objective at this time is the development of equipment of greater detection sensitivity which will permit studies of N15 at its low natural-abundance level. With C13, substantial success has already been achieved in this way, and studies have been made of bile acids, alkaloids, vitamins, substances with marijuana-like activity, anticoagulants, enzymes, and pentacyclic triterpenoids. It is expected that comparable results will be obtained with N15 with the spectrometer just made available. This equipment features Fourier- transform operation with exceptionally large sample diameters (25 mm) and a 45-kilogauss field.